1. Technical Field
The present invention relates to wireless networks, and, more particularly, to managing transmission power between mobile stations and base stations.
2. Description of Related Art
a. CDMA Networks Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1×RTT networks” (or “1× networks” for short), which stands for “Single Carrier Radio Transmission Technology.” These networks typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Typical CDMA networks include a plurality of base stations, each of which provide one or more wireless coverage areas, such as cells and sectors. When a mobile station is positioned in one of these coverage areas, it can communicate over the air interface with the base station, and in turn over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the mobile station conduct these communications over a frequency known as a carrier, which may actually be a pair of frequencies, with the base station transmitting to the mobile station on one of the frequencies, and the mobile station transmitting to the base station on the other. This is known as frequency division duplex (FDD). The base-station-to-mobile-station link is known as the forward link, while the mobile-station-to-base-station link is known as the reverse link.
Furthermore, using a sector as an example of a wireless coverage area, base stations may provide service in a given sector on one carrier, or on more than one. An instance of a particular carrier in a particular sector is referred to herein as a sector/carrier. In a typical CDMA system, using a configuration known as radio configuration 3 (RC3), a base station can, on a given sector/carrier, transmit forward-link data on a maximum of 64 distinct channels at any given time, each channel corresponding to a unique 64-bit code known as a Walsh code. Of these channels, typically, 61 of them are available as traffic channels (for user data), while the other 3 are reserved for administrative channels known as the pilot, paging, and sync channels.
When a base station instructs a mobile station—operating on a particular sector/carrier—to use a particular traffic channel for a communication session, such as a voice call or a data session, the base station does so by instructing the mobile station to tune to a particular one of the 61 Walsh-coded traffic channels on that sector/carrier. It is over that assigned traffic channel that the base station will transmit forward-link data to the mobile station during the ensuing communication session. And, in addition to that Walsh-coded forward-link channel, the traffic channel also includes a corresponding Walsh-coded reverse-link channel, over which the mobile station transmits data to the base station.
b. Reverse-Link Transmission-Power Management
i. The Power Control Bit (PCB) and the Ratio Eb/Nt 
In CDMA networks, the transmitting power of a mobile station on the reverse link of a traffic channel at any given moment is based on a number of factors, two of which are known as the power control bit (PCB) and the ratio Eb/Nt. The PCB is a bit (0 or 1) that the base station sends to the mobile station on the forward link at a high frequency, on the order of 800 times per second (i.e. once every 1.25 milliseconds (ms)). The mobile station repeatedly responsively adjusts its transmission power to the base station on the reverse link. Typically, if the base station sends a 0, the mobile station will decrease the power by a set decrement, such as 1 dB, whereas, if the base station sends a 1, the mobile station will increase the power by a set increment, which may also be 1 dB. Thus, using these numbers, the mobile station's reverse-link transmission power would change by plus or minus 1 dB every 1.25 ms.
Each such 1.25-ms cycle, a typical base station determines whether to transmit a PCB equal to 0 or 1 to a given mobile station by comparing (i) a signal-to-noise ratio that the base station computes for that mobile station with (ii) a stored threshold value for that signal-to-noise ratio that the base station maintains on a per-mobile-station basis. This ratio is generally known and referred to herein as Eb/Nt, while the threshold is referred to herein as the Eb/Nt threshold. Eb/Nt compares the strength at which the base station is receiving the reverse-link signal from the mobile station (Eb for “energy per bit”) with the strength at which the base station is receiving signals from all other sources on the frequency of the sector/carrier on which that mobile station is operating (Nt for “noise”). Eb/Nt, then, is a signal-to-noise ratio for the reverse-link part of the traffic channel. As stated, the base station typically computes Eb/Nt at the same frequency at which it transmits the PCB, which again may be once every 1.25 ms.
Thus, in typical operation, for a given mobile station (and in fact for each mobile station that the base station is serving), every 1.25 ms, the base station compares the most recent computation of Eb/Nt for that mobile station with the Eb/Nt threshold for that mobile station. If Eb/Nt exceeds the threshold, then the base station is receiving a strong enough signal from the mobile station, and thus it transmits a PCB of 0, causing the mobile station to reduce its reverse-link transmission power. If, on the other hand, the computed Eb/Nt is less than the threshold, the base station is not receiving a strong enough signal, and thus it transmits a PCB of 1, causing the mobile station to increase its reverse-link power. Thus, the reverse-link power on the traffic channel typically stabilizes to a point that achieves an Eb/Nt value (as measured at the base station) that is near the Eb/Nt threshold. And this threshold can be changed during operation.
ii. Reverse-Link Frame Error Rate (RFER)
In CDMA networks, data is transmitted from the mobile station to the base station (and vice versa) in data units known as frames, which typically last 20 ms. Some frames received by the base station contain errors as a result of imperfect transfer from the mobile station, while some do not. The reverse-link frame error rate (RFER) is a ratio, computed on a per-mobile-station basis by the base station, of the number of error-containing frames that the base station receives from a given mobile station to the total number of frames that the base station receives from the given mobile station, over a given time period. Note that the RFER often also takes into account frames that are not received at all by the base station. And other things being more or less equal, the more power the mobile station uses to transmit to the base station, the lower the mobile station's RFER will be.
More particularly, at approximately the same frequency at which the base station is receiving reverse-link frames (i.e. once every 20 ms) from a given mobile station, the base station computes a RFER for that mobile station over some previous number of frames, which may be 20, 100, 200, or some other number. Thus, the base station essentially computes a RFER for some rolling window of previous frames. And each time the base station computes the RFER for that mobile station, the base station compares that computed value with a threshold: a sector/carrier-level parameter often referred to as the “RFER target,” which may be around 2%.
If the RFER for that mobile station exceeds the RFER target for the sector/carrier, the base station is receiving too many error-containing frames and/or missing too many frames from that mobile station, and thus the base station will responsively increase its Eb/Nt threshold related to that mobile station. In the short term, this will result in the base station's computed Eb/Nt for that mobile station falling below the increased threshold, which in turn will result in the base station repeatedly sending PCBs equal to 1 to the mobile station. This, in turn, will result in the mobile station increasing its reverse-link transmission power, which will then typically stabilize at a level that will result in the base station computing an Eb/Nt for that mobile station that is close to the new, increased Eb/Nt threshold that the base station is maintaining for that mobile station, and perhaps result in an acceptable RFER for that mobile station.
If, on the other hand, the RFER falls below the RFER target, the mobile station may be using excessive power for transmitting on the reverse-link—in essence, the base station may be receiving a signal from that mobile station that may be considered too strong, perhaps at the expense of that mobile station's battery life, and perhaps creating excessive noise from a single mobile station on the sector/carrier. If that situation holds for a specified period of time, the base station may decrease the Eb/Nt threshold that the base station is maintaining for that mobile station, resulting in the base station's computed Eb/Nt repeatedly exceeding the decreased threshold. This, in turn, will result in the base station repeatedly sending PCBs equal to 0 to the mobile station, which will result in the mobile station decreasing its reverse-link transmission power, which will then typically stabilize at a level that will result in the base station computing an Eb/Nt that is very close to the new, decreased Eb/Nt threshold.
Thus, the base station's repeated RFER calculation for the mobile station and comparison with the RFER target for the sector/carrier causes the base station to iteratively adjust its Eb/Nt threshold corresponding to the mobile station. In turn, the base station's even-more-frequent calculation of Eb/Nt and comparison with its current Eb/Nt threshold for the mobile station causes the base station to iteratively send PCBs of 0 (for less power) or 1 (for more power) to the mobile station, which then causes the mobile station to adjust its reverse-link transmission power on the traffic channel. This entire back-and-forth calibration process is conducted in an attempt to keep the RFER calculated by the base station and associated with the mobile station at or below what is deemed to be an acceptable threshold, which again may be around 2%.
Note that different situations may present themselves on a given sector/carrier at different times. For one, the number of mobile stations using traffic channels can vary between just a few, such as 10, to a larger number, such as 30, and perhaps approach the upper bound of 61 (assuming RC3). And, as stated, the power that the mobile stations use for transmission to the base station can vary. In particular, variables such as terrain, weather, buildings, other mobile stations, other interference, and distance from the base station can affect the RFER that the base station experiences for a given mobile station, and thus the amount of power the mobile station uses on the reverse link. Using too much power can drain battery life, and it may sometimes be the case that a mobile station reaches its maximum transmission power and still cannot achieve an acceptable RFER, in which case it may not be able to communicate with the base station.
Note that, in some implementations, a ratio other than Eb/Nt may be used. In particular, each mobile station, when operating on a traffic channel, may also transmit on the reverse-link on what is known as a reverse pilot channel. The base station may then compute a ratio known as Ec/Io for that mobile station, which would be a ratio comparing (a) the power level at which the base station is receiving the reverse pilot channel (“Ec” for “energy per chip”) and (b) the power level at which the base station is receiving all transmissions (“Io”) on the frequency (sector/carrier) on which the mobile station is operating (including the reverse pilot channel).
The base station would then operate with respect to Ec/Io as described above with respect to Eb/Nt. That is, the base station would maintain an Ec/Io threshold for each mobile station, and repeatedly compare the measured Ec/Io with the Ec/Io threshold, and send PCBs equal to 0 or 1, causing the mobile station to either decrease or increase its reverse-link transmission power. The base station would also adjust the Ec/Io threshold as described above with respect to the Eb/Nt threshold, in an effort to keep each mobile station at or just below the RFER target.
iii. Reverse Noise Rise (RNR)
As stated, in general, interference can be—and often is—present on the reverse link of a given sector/carrier. That is, on the given sector/carrier, a base station will receive transmissions not only from mobile stations that are operating on that sector/carrier, but will also often receive transmissions on that frequency from other mobile stations, other transmitting devices, and/or any other sources of interference on that frequency in that area. At a given moment, the sum total of what a base station is receiving on a given sector/carrier (i.e. a given frequency)—including transmissions from mobile stations operating on that sector/carrier, as well as from all other sources—is known as the “reverse noise” on that sector/carrier.
Quite frequently (e.g., once per frame (i.e. once every 20 ms)), base stations compute a value known as “reverse noise rise” (RNR) for a given sector/carrier, which is the difference between (i) the reverse noise that the base station is currently detecting on the sector/carrier and (ii) a baseline level of reverse noise for the sector/carrier. Thus, the base station computes how far the reverse noise has risen above that baseline.
For the baseline level, CDMA networks may use a value such as the lowest measurement of reverse noise on the sector/carrier in the previous 24 hours, or perhaps an average of the 24-hour lows over the previous week, or some other value. Incidentally, some networks, including Evolution Data Optimized (EV-DO) networks, may periodically use what is known as a silent interval, which is a coordinated time period during which mobile stations know not to transmit anything to the base station. The base station can then measure whatever else is out there. In that case, the baseline level would correspond to the amount of reverse noise when the sector/carrier is unloaded. And other reverse-link-noise levels could be used as a baseline.
Other things being more or less equal, the lower the RNR is at a given moment, the more favorable the RF environment is for communication between mobile stations and the base station at that time. Correspondingly, the higher the RNR, the less favorable the RF environment is. Also, a low RNR generally corresponds to a sector/carrier being lightly loaded, in other words that is supporting communications for a relatively low number of mobile stations. A high RNR, as one might expect, generally corresponds to a sector/carrier being heavily loaded, in other words that is supporting communications for a relatively high number of mobile stations.